Multi-component closures for vessels such as flasks, vials, tubes and the like have conventionally been manufactured by injection or compression molding of the individual components and subsequently combining the individual parts in one or more separate processes. For example, in the manufacture of tube closures, a septum for penetration by a needle is stamped out of an elastomeric sheet and adhered to a preformed cap portion. Additional components, such as a foil lining, may then be added.
Such processes are inefficient from a processing standpoint and also result in material waste at each step. In all injection molding machines, some polymer remains in the sprue after the mold is closed, leaving a projecting piece or tab which must be removed after the product is ejected from the mold. Often, the tabs are simply discarded, or for reasons of economy, may be recovered and recycled. In either case the inefficiency and additional cost which result lead to reduced productivity from the mold.
In compression molding, an apparatus which resembles a waffle iron is used wherein melted polymer fills all mold cavities without passing through gates and runners. When all the cavities are filled, the mold is closed and heat and pressure are applied from a hydraulic press. While this process eliminates gates and runners, a surplus of polymer must be used to ensure total cavity fill. The heat and pressure cause the polymer to fill the cavity and cause spillage out into overflow grooves. Polymer in the grooves, like the tabs in injection molding, must be recycled or discarded. Compression molding, like injection molding, is inherently a batch process but is less efficient than injection molding, because a separate machine is required to separate the individual articles from the single large "waffle" which comes out of the compression mold.
Rotary compression molding is a recent development in which individual molds on the track of a continuously revolving platform are individually charged with a melt of thermoplastic material. Compression and heating of the thermoplastic in the mold by a plunger forms the desired article. The mold-plunger unit advances along the track and is opened when the article has solidified.